1) Noun 1.
usability - the quality of being able to provide good service
Based on WordNet 3.0, Farlex clipart collection. © 2003-2008 Princeton University, Farlex Inc.
2) Usability is defined by five quality components:
•Learnability: How easy is it for users to accomplish basic tasks the first time they encounter the design?
•Efficiency: Once users have learned the design, how quickly can they perform tasks?
•Memorability: When users return to the design after a period of not using it, how easily can they reestablish proficiency?
•Errors: How many errors do users make, how severe are these errors, and how easily can they recover from the errors?
•Satisfaction: How pleasant is it to use the design?
There are many other important quality attributes.http://www.useit.com/alertbox/20030825.html